You know, Alfbennet is right. You could get one of those sterilite bins from Walmart. They come in all sizes, and even probably in a 20 or 30 gallon.
You know what I'm talking about alyssaa? They are rectangular translucent storage containers in the housewares department. They come with lids, but you don't need to put the lid on it.
Walmart does have HOB (hang on back) filters for a variety of sizes. I know I've seen some for even up to 30-60 gallons at my Walmart.
One of our AC members told me she uses those sterilite storage bins at times for temporary tanks, just rinse with clean water, no soap.
Fill with the right temperature (same as the water they are in) water and use the dechlorinator.
Hang the filter on it, and unless your house is really cold you don't need a heater. You don't even need to put gravel in it really, but the little cave you have would be good. They'll soon outgrow it, though.
You will still need to do regular water changes, and you could get a turkey baster, clean and new, and use it to suck out uneaten food and poo. Your dad might have a syphon that he uses to change the water in his tanks, and if so he may let you use it.
When you do water changes it's important to use clean containers. If you use plastic, like a bucket or big bowl to change the water, you need to use one that has never had soap in it. Plastics can retain some of the harmful ingredients that are in soap, and that can make your fish sick or even kill them.
It's also important that you make sure you don't have any residue of soap, or lotions, or perfumes, or other chemicals or substances on your hands and arms when you do water changes, or stick your hands in the tank with your fishies.
I always just rinse and rinse my hands and arms, scrubbing and rubbing to remove any lotion or soap residue. If I thought my hands had oils, or maybe if I think my hands aren't clean, then I do use soap to wash up, but I make sure I rinse and rub and get all the soap residue off.
You also want to be careful and not spray any aresols in the room, unless you cover the tank for long enough for it to dissipate. Any kind of chemical exposure can be fatal, and that includes air fresheners, perfumes, bug spray (especially!!) and the like.